1. Field of the Invention
The present invention generally relates to a method of and apparatus for continuous, near real-time measurement of airborne metals emitted into the atmosphere by furnaces, incinerators, boilers, and other combustors and detected by inductively coupled plasma atomic emission spectrometry (ICP-AES). This apparatus can provide emission data in the expeditious time frame required to facilitate pollution control efforts. The invention specifically relates to the introduction of air samples to an ICP-AES apparatus whereby sample loss is minimized, a sampling interface is employed so that isokinetic sampling of the air sample from the combustor can be utilized and a computer is coupled to the ICP-AES apparatus which provides an alarm or feedback signal if a maximum emissions level is reached.
2. Prior Art
Present and future regulatory restrictions on the rates and composition of hazardous air pollutant (HAP) emissions are creating a need for faster, more sensitive, and more reliable methods of monitoring these emissions. The traditional methods for monitoring airborne metals, EPA Methods 5 and 29, are both labor-intensive and time-consuming.
Determination of airborne metals presently requires a sample collection step, typically on a cellulose or glass fiber filter. The air to be tested is sampled for many hours. The filter is then transported to a laboratory where the filter media is chemically digested to recover and dissolve any captured metal elements. The digest is analyzed by an appropriate spectroanalytical technique such as atomic absorption spectrometry. One problem associated with this method is that the sensitivity is directly dependent on the duration of the sample collection process. Sample air has to be collected over a long time to meet the low detection levels required. This approach is insensitive to transient events since it involves time-averaging over a long time. Since the analytical result is available only after the fact, it has limited value in terms of process control aimed at pollution prevention.
There have been several approaches to continuous emissions monitors for measuring HAP metal concentrations such as: X-ray fluorescence spectrometry, laser assisted spark spectrometry, and ICP-AES. For many approaches speed and sensitivity are mutually exclusive goals. With X-ray fluorescence the sample collection process is relatively slow. X-ray fluorescence also requires expensive equipment which is designed to measure only for specific pollutants. In the laser assisted spark spectrometry technique, the sensitivity is limited for several important HAP metals. Another disadvantage of the laser assisted spark spectroscopy is it is not as accurate as the ICP-AES apparatus. The laser pulses and takes minute, pinpoint measurements of the airborne metals, whereas the ICP-AES apparatus measures larger, more representative volumes of air. Also, a reliable standardization scheme for the laser assisted spark spectroscopy has not been shown.
An apparatus for in situ detection of airborne metal aerosols was reported that involved the suction of ambient air through a plasma sustained in a quartz confinement tube mounted inside a helical induction coil. Aqueous standards were used for standardization, allowing for such factors as nebulizer efficiency and atmospheric water vapor content. A major disadvantage of the sample introduction approach used here is that the entrained analyte is dispersed throughout the plasma rather than concentrated in the central, or analytical region where excitation processes are optimum. Thus, the analyzer is insensitive to lower concentrations of airborne metals.
A method for detection and measurement of the metallic aerosol concentrations in atmospheric air was disclosed which involved channeling the sample through direct suction to the plasma. A disadvantage with this method is that the sample air flow has to be physically restricted, since the suction would cause the sample air to flow through the plasma too fast for an accurate spectroscopy reading of the emitted metal aerosols in the sample air. For standardization of the ICP-AES apparatus a relationship between the aqueous solutions of certain metals and their aerosol equivalents was also disclosed. A disadvantage with this standardization method is that it was based on an inefficient nebulization process. The amount of metal in an aerosol is calculated based on this "known" inefficiency.
Instrumentation capable of real-time and accurate detection of airborne metals emitted from industrial combustors and incinerators is not available presently. There has been interest in recent years in the development of instrumentation and methodology to permit direct sampling and analysis of airborne metals. This instrumentation and methodology would allow continuous, real-time monitoring of these metals. Real-time monitoring has an advantage in that results are available instantaneously and can be of significant value for controlling pollution in the time frame that it is occurring. Real-time data can be used as feedback in a process control loop designed to facilitate optimum operation of an incinerator or other combustor while promoting operation that meets environmental regulations.